A) Probability densities of GFP molecules, for three versions of the O
promoter model. Densities were generated by accumulating statistics in runs 250000 cell cycles in duration. (Solid line) Fully discrete model. (Dashed line) Fully continuous model. (Dash-dotted line) Hybrid model: the DNA binding states are represented by discrete variables, while all other species are continuous. All three methods produce virtually identical probability distributions. B) Probability density for species D8, for the continuous model. The hybrid and fully discrete methods produce identical distributions in which D8 spends 99.996% of the time at zero. This histogram shows that the continuous model artificially allows negative numbers (see inset time series).